For ceramic PTC resistors, which include components having a resistor with a positive temperature coefficient or so-called PTC elements, no conventionally used, temperature-stable electrodes manufactured of precious metal are suitable. These cannot form resistive contact between the ceramic material and the metallic electrodes. Therefore, PTC elements with (internal) electrodes manufactured of precious metal have an inordinately high resistance. The non-precious metals suitable for electrode material, however, generally do not withstand the sintering process that is necessary for the construction of multi-layer components.
From the publication. DE 19719174 A1, a ceramic PTC resistor in multi-layer design equipped with electrode layers containing aluminum is known to the art. These layers form a resistive contact with the ceramic material and can be sintered at temperatures of up to 1200° without incurring damage. The disadvantage in this multi-layer PTC component, however, is the fact that the aluminum partially diffuses from the electrode layers into the ceramic material, thereby impairing the component properties in the medium or long term or even making the component unusable.
From the publication DE 100 18 377 C1, a PTC component is known to the art that is a multi-layer component consisting of stacked ceramic layers and which is sintered or re-tempered in an atmosphere with high oxygen content. The PTC component contains, internal electrodes with tungsten. Tungsten does withstand the sintering process.
However, sintering or subsequent tempering at high oxygen partial pressure entails the danger of the oxidation of the internal electrodes, which results in PTC components with high resistance; this is not desirable.
Sintering in an oxygenic atmosphere, on the other hand, is necessary in order to form the grain boundary-active layers of the PTC ceramic material (on the basis of doped BaTiO3) during the cooling process. This results in the situation that at a certain temperature the resistance of the ceramic material increases erratically, depending on the precise composition of the ceramic material.